The invention relates to a process for preparing articles with a high tensile strength and a high modulus starting from a polyacryonitrile solution.
Synthetic fibres based on polyacrylonitrile are well known and are used on a large scale in the textile industry. For this use the fibres are required to meet high standards in respect of, for instance paintability of the colour fastness, while the tensile strength and modulus of the fibres are less important. For this use, therefore, fibres having a tensile strength of 0.3-0.5 GPa and a modulus of 5-8 Gpa are generally applied, see `International Conference on man-made Fibres for developing countries (1982), Sasmira Bombay pp 1-4` by B. von Falkai.
In addition there is a large and growing market for the so-called technical-fibres based on polyacrylonitrile, such as for cement reinforced with these fibres, the use in brake shoes and other materials of friction etc. For these uses the fibres must have a higher tensile strength and a higher modulus.
Another major field of application for technical polyacrylonitrile fibres is the preparation of carbon fibres and of the so-called POX (=partly oxidized) polyacrylonitrile fibres, where technical polyacrylonitrile fibres are subjected to thermal after-treatment, for instance to above 2000.degree. C. in the preparation of carbon fibres and to about 250.degree. C. in the preparation of the POX fibres. It has been found that for this use, too, improved mechanical properties of the polyacrylonitrile fibre, particularly high tensile strength and modulus, lead to carbon and POX-PAN fibres with better performance, see M. Santappa in `Journal Indian Chem. Soc. 59 (1982) pp. 321-328`.
Though numerous methods have already been proposed for preparing technical polyacrylonitrile fibres having high strength and modulus, it has been found that by applying the known commercial processes only fibres can be prepared having tensile strengths to about 0.8 GPa and moduli to about 16 GPa, see, for instance, Technisch Informatie Bulletin no. 0198 of Mische, J and no. 0198b of Sassenrath, B., issued by Hoechst on 30.11.1982.
As known in the art, mainly two spinning methods are used in preparing polyacrylonitrile fibres, viz. the so-called wet spinning and dry spinning. Another spinning method known in the art, the so-called melt spinning used for various polymers, is hardly feasible in the spinning of polyacrylonitrile, because at elevated temperature the polymer is unstable in consequence of intramolecular cyclization. In the dry as well as wet spinning processes the polyacrylonitrile is dissolved in a suitable medium, the solution is pressed through openings while filaments are being formed, upon which these filaments are dried in heated gas (dry spinning) or passed into a coagulating bath (wet spinning). In this operation the solvent is removed from the filament, upon which the product is ready for further treatment, such as cutting, twisting, afterstretching. It is known that the spinning conditions influence the mechanical properties of the fibres finally obtained after the stretching. Numerous proposals have therefore been made to improve the mechanical properties of the finally resulting fibres by applying specific conditions, for instance special coagulation methods, special additives to and concentrations of the solution to be spun. Generally, however, these proposals have resulted in only marginal improvements of, among other things, tensile strength and modulus.
As known in the art, the spun filaments obtained after removal of the solvent only have a low strength and modulus, and these filaments must be subjected, for technical applications, to afterstretching. For this afterstretching process, too, numerous proposals have already been made, such as the addition of plasticizers, afterstretching in a plurality of steps under specific conditions or not, in order thus to obtain filaments with high tensile strength and modulus. These proposals, too, have generally resulted in only marginal improvements. See for instance: A. I. Stoyanov, Journal of Appl. Pol. Science, 27, page 235. W. Sarmadjieva et al., JSDC, 97 (1981) page 465. A. I. Stoyanov, Journal of Appl. Pol. Science, 24, page 583. S. Minami, Appl. Polymer Symposium, 25 (1974) page 145. R. B. Beevers, Journal of Appl. Pol. Science 9 (1965) page 1499.
It has already been proposed (see GB-A-2.018.188) to prepare polyacrylonitrile fibres having a relatively high tensile strength (about 1 GPa) and modulus (about 12 GPa) by carrying out the spinning as well as the stretching under highly specific conditions. According to this known process a polyacrylonitrile is dissolved in an aqueous thiocyanate solution, the solution is spun in an aqueous coagulating medium to form filaments, which are then subjected to a first stretching process, subsequently washed, subjected to a second stretching process in water at elevated temperature, and then subjected at high temperature in a zone under steam pressure to a third stretching process. A disadvantage of this process is that it is very laborious, while the strength and modulus of the resulting fibres are indeed higher than in the processes applied for commercial purposes, but yet insufficient for a number of technical uses.
It is further known to prepare fibres having a very high tensile strength and modulus starting from solutions of polyethylene with a high molecular weight, see U.S. Pat. Nos. 4,344,908; 4,422,993 and 4,430,383. According to the process described in U.S. Pat. No. 4,344,908, to this end a relatively dilute solution of the polyethylene is spun, the resulting filament cooled to form a gel filament, and the solvent-containing gel filament stretched at increased temperature. According to the process described in U.S. Pat. Nos. 4,422,993 and 4,430,383, to this end a solution of high-molecular polyethylene is spun, the solvent largely or partly removed if so desired, and the gel filament subjected at a specific temperature to a high degree of stretching related to the molecular weight. In applying these known processes it has been found that as the molecular weight of the polyethylene increases, the moduli that can be reached, but particularly the tensile strengths that can be reached, will be higher. By applying this known process polyethylene-based fibres can therefore be prepared having tensile strengths far beyond 1.2 GPa and moduli of more than 20 GPa.
In the said U.S. Pat. No. 4,344,908 it is stated that the process can be generally applied to materials that can be processed by solution spinning to form filaments, for instance also polyacrylonitrile. It has now been found, however, that although in applying this process for the spinning and stretching of high molecular polyacrylonitrile a fibre with a substantially higher modulus is obtained, viz. about 15 GPa, the tensile strength attainable, however, increases only slightly, for instance to 0.7-0.8 GPa. Prima facie this known process consequently does not seem economically attractive for polyacrylonitrile, considering the extra effort that must be put forth in respect of the polymerization of high molecular polyacrylonitrile, the lower concentration of the solution to be spun and the lower process efficiency related thereto, and the problems inherent in the handling of high molecular solutions to be spun.